Why is that electron moving?

I'm working on a rather big chemistry assignment. Right now I'm looking at a reaction mechanism. The first step is illustrated in the attachment. The big thing with the loop represents a glucose molecule, and on the bottom there is some Pd+Bi attached to active coal as an adsorbent (if i'm using the wrong terms, sorry, I never got any other chem education than in Dutch).
Now, it says that an electron is displaced from the Pd to the Bi, but it seems to assume that I know why. It's not an actual part of the assignment, and I don't really need it, but I'd like to know why that electron is moved. I've been thinking about electronegativity (Pd 1.4, Bi 1.7), but that can't be all, I think.

Bi is commonly used as a chemical (or electronic) promoter in Pd or Pt catalysts. Typically, the function of the (chemical) promoter is to improve activity of the catalyst, and it usually does this by altering the charge density at the surface of the catalyst. It is entirely possible that there is some charge transfer mechanism related to the electronegativity, but I don't know much about catalysis to say anything specific about your reaction.

It might help if you cite the original source for the reaction mechanism you are studying.